cxl/test: Add Get Supported Features mailbox command support

Add cxl-test emulation of Get Supported Features mailbox command.
Currently only adding a test feature with feature identifier of
all f's for testing.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Li Ming <ming.li@zohomail.com>
Link: https://patch.msgid.link/20250220194438.2281088-4-dave.jiang@intel.com


Signed-off-by: Dave Jiang <dave.jiang@intel.com>